1. Field of the Invention
The present invention relates to a developer sheet which is useful in providing visible images through reaction with a color precursor and can provide transparencies or reproductions having a glossy finish. More particularly, the present invention relates to a developer sheet which is capable of providing a controlled degree of gloss ranging from matte to high gloss, and may take the form of a transparency having minimal surface defects.
The developer sheet of the present invention can be used in conjunction with conventional pressure-sensitive copy paper or photosensitive imaging systems employing microcapsules to provide visible images upon contact with a color precursor which is image-wise released from the microcapsules. The imaging system according to the present invention may either be a transfer imaging system or a self-contained imaging system.
2. Description of the Prior Art
Pressure-sensitive copy paper is well known in the art. It is described in U.S. Pat. Nos. 2,550,446; 2,712,507; 2,703,456; 3,016,308; 3,170,809; 3,455,721; 3,466,184; 3,672,935; 3,955,025; and 3,981,523.
Photosensitive imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,416,966 to The Mead Corporation as well as copending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. Image-wise exposure hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to an uniform rupturing force by passing the sheet through the nip located between a pair of pressure rollers.
U.S. Pat. No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer sheet prior to being subjected to the rupturing force. Upon passing through the pressure rollers in contact with the developer sheet, the microcapsules rupture and image-wise release the internal phase whereupon the color-former migrates to the developer sheet where it reacts with a dry developer and forms a color image. The imaging system can be designed to reproduce monochromatic or polychromatic full color images.
U.S. Pat. No. 4,440,846 discloses a so called "self-contained" imaging system wherein both the image-forming agent and the developer material are located on the same substrate. In the system according to U.S. Pat. No. 4,440,846, the image-forming agent is encapsulated in a layer of pressure rupturable capsules, and the subsequent exposure and capsule rupture causes the image-forming agent to contact and react with the developer to produce an image on the substrate.
In those applications in which the aforementioned pressure-sensitive and photsensitive imaging systems are used to reproduce graphic or picture images, a high degree of gloss is often desired in the reproduction. In particular, where a transparency is desired, the reproduction must transmit light effectively. These objectives are difficult to achieve using conventional approaches. Because the reaction between the developer and the color-former occurs principally at the surface of the developer, finely divided developers such as resin grinds are developers of choice. They provide a large reactive surface area to react with the color-former and provide high optical density. However, these compositions are made of particles which tend to scatter light and make the image appear dull.
A further problem which occurs when producing transparencies is that the transparency suffers from surface defects commonly referred to as "fish eyes". These defects are caused by the developer layer not forming a continuous uniform film onto the transparency substrate. The discontinuities on the transparency surface results in an inferior final product.
Although attempts have been made in the art to correct some of the above described problems, until the present invention, a solution has not been proposed which can easily remedy these defects.
For example, U.S. Pat. No. 4,554,235 discloses a technique for the production of high gloss images. According to this patent, the developer layer is overcoated with a discontinuous layer of a thermoplastic polymeric pigment. After exposure of an imaging sheet containing thereon a color-forming agent to actinic radiation, the imaging sheet is assembled with the developer sheet and subjected to pressure. The color-former is released from the microcapsules, passes through the layer of polymeric pigment, and reacts with the developer. Thereafter, the thermoplastic pigment is subjected to heat and/or pressure which causes the pigment to coalesce into a transparent continuous thin film which imparts a glossy finish to the image.
An alternative approach in producing high gloss images is disclosed in U.S. patent application Ser. No. 073,036, filed July 14, 1987. According to the teachings of this application, the developer materials utilized are film forming, thermoplastic resins. The resins are provided on a support as finely divided particles. The developer sheet is used in a conventional manner except that, after or simultaneously with development, the developer particles are subjected to heat and/or pressure to cause them to coalesce into a film. According to the teachings of this application, preferred resins of use are phenolic resins.
While the developer materials described in U.S. patent application Ser. No. 073,036 are satisfactory the temperatures desired to effect coalescense of the resins limit the number of resins which may be used and, as such, lower coalescence conditions would be desirable. Furthermore, some of these resins will not uniformly coat synthetic substrates. Further, when producing a transparency according to this invention, the resultant product still suffers from discontinuities accumulated on the surface of the transparency.
Accordingly, there exists a need in the art for a developer sheet which may be utilized to produce a controlled degree of glossing (transparentizing when the substrate is a transparent film) ranging from matte to high gloss while minimizing surface defects. There further exists a need in the art for a developer sheet which utilizes a developer material which coalesces into a continuous thin film wherein the coalescing process takes place at reduced temperatures or for shorter glossing periods.